Lecture No (3) Latex

Transcription

1 Lecture No (3) Latex

2 Rubber Latex The latex is a mixture of compounds consists of special structures called laticifers, which have many compositions for different types, but typically contains water, sugars, enzymes, etc. Therefore latex is defined as a dispersion of polymer in an aqueous medium (water) or in non-aqueous medium in which the polymer is insoluble in it. That is a white, milky fluid that is held in cells that find in the inner layers of the trees. The latex has two phases which includes: 1- The dispersed phase or discontinues phase consisting of small particles of polymers (particle size < 5 micrometer in diameter). 2- The continuous phase or (a dilute aqueous solution). Gutta-Percha Rubber The other source of natural rubber is gutta-percha. This type of rubber has all trans structure and has very different physical properties from natural rubber that obtained from hevea brasiliensis trees. Gutta-percha get off from plantations in Malaysia and used for golf balls, undersea cables, and this material choice for packing teeth in root. Classification of Latex The latices can be classified into many types which include: 1- Natural latex obtains from the trees in plantations. 2- Synthetic latex produced from emulsion polymerization process. 3- Artificial latex produced by dispersing the appropriate bulk polymer in an aqueous dispersion medium. 4- Modified latex produced by modification of existing type of latex by (grafting, cross linking, etc). Natural Rubber Latex Concentrate The natural rubber (NR) latex obtained from the plantation is called fresh or field latex. The field latex contains about 33% NR and thus not economical to be used to make a latex products. The rubber content of the field latex need to be increased before it is used in making latex products. This is achieved by doing 1

3 concentration process to the field latex. There are few methods that can be used to concentrate natural rubber latex: 1- Concentration by creaming 2- Concentration by centrifugation 3- Concentration by evaporation Latex Preservations The preservation of latex is necessary to prevent micro-organism from attacking the non-rubber components of latex concentrated. Concentrated latex has lower colloidal stability and bad odor, when attacked by micro-organism. Preservative Materials Concentrated latex is preserved for long-term storage. The most popular preservative is ammonia. There are two types ammonia preservation systems: 1- Low ammonia latex (LA latex): Added 0.2 % ammonia for short term preservation suitable for certain applications such as latex foam, requires low ammonia content in order for to gel properly, 2- High ammonia latex (HA latex): Added % of ammonia, this process called secondary preservatives are added to LA latex for long term preservation (storage). Latex Properties Natural rubber latex concentrates are highly specified materials and characterize by many properties that are includes: 1- Dry Rubber Content (DRC). 2- Total Solid Content (TSC). 3- Alkalinity. 4- Potassium Hydroxide (KOH) Content. 5- Mechanical Stability Time (MST). 6- Sludge Content. The properties of the latex are of interest because of: 1- To gain understanding of the physical and chemical nature of the latex. 2

4 2- To determine the fitness of the latex for a particular applications. 3- To assure the general quality of the latex. Latex Tests The test methods required to evaluate the latex properties includes: 1. Tests related to chemical composition. 2. Tests related to colloidal stability. 3. Tests related to physical properties. Latex Processing Steps Diagram Latex Processing Raw Latex Latex Compounding Latex Curing Vulcanize latex / End Rubber Latex Product Latex Compounding Compounded latex is defining the process of addition of chemical materials to the raw latex. Most chemical materials added to the latex in solid form, therefore these materials need to be dispersed in water first before added to the latex. The elastic properties are produced by mix raw rubber latex with specific additives during rubber compounding. The chemicals materials dispersion and prepared by grinding them together with the dispersing in water. The chemicals materials used in latex compounding can be divided into three general classifications: 1. Surface active agents (surfactants). 2. Liquid phase modifier. 3

5 3. Elastomer or rubber phase modifier. Latex Compounding Formulation General latex compounding formulation from: Elastomer/ Raw latex 100 KOH 0.3 Sulphur 0.5 Zinc Oxide 0.25 Accelerator 0.75 Antioxidant 0.5 Fillers 15 Total Note: Parts per hundred rubbers (pphr), all the ingredients used in a compound formulation are given in amounts based on a total of 100 parts of the rubber. Latex Curing Curing of latex define the process of introducing cross-links in the rubber molecules. The term vulcanization used to describe cross-linking process involving sulphur as cross-linking agent. Two types of vulcanization in latex: 1- Pre-vulcanization: when the vulcanization is done while the latex is still in liquid state. 2- Post-vulcanization: when the vulcanization is done on latex film. Latex that has been pre-vulcanized referred as pre-vulcanized latex or PV latex for pre-vulcanized natural rubber (PVNR). Appearance of PVNR is very similar to un-vulcanized natural rubber. Because of maintained fluidity, cross-linking only takes place in each individual particle and particles in PVNR have the same shape, size and size distribution as those in un-vulcanized latex. Latex Dipping Process Dipping process is used to make thin and hollow latex products such as gloves, catheters and toys, these products are called dipped products. 4

6 In principle the dipping process involves dipping cleaned formers into the compounded latex. Latex film will be formed around the former and latex film product obtained by drying and curing (pre-vulcanized or post-vulcanized compound) the films. Latex dipping process can be classified into three methods: 1- Straight or simple dipping: when no stabilize agency is used to form the latex films. 2- Coagulant dipping: when a direct coacervate is used to promote latex film formation. Note: Coacervate meaning a colloid-rich viscous liquid phase that may separate from a colloidal solution on addition of a third component. 3- Heat-sensitized dipping: when a heat sensitizer is used to promote latex film formation. Rubber Requirements 1- High Molecular Weight: rubber elasticity is due to the coiling/uncoiling of molecular chains. 2- Use Temperature must be above glass transition temperature (Tg): to allow for molecular chains motion. 3- Amorphous in its unstretched state: the crystals would hinder coiling/uncoiling of molecular chains. 4- Molecular Chains tied together: to prevent flow of molecular chains, traditionally occur through crosslinking and entanglements. Note: All polymers above glass transition temperature (Tg), act as rubbers due to entanglements. However, when the entanglements break, the polymer will begin to flow. 5